1. Technical Field
The invention relates to a projection zoom lens of a three-group configuration incorporated into a projection display device and a projection display device using the projection zoom lens. More particularly, the invention relates to a projection zoom lens for projecting a luminous flux that contains video information from a light valve of a DMD (digital micromirror device) display device onto a screen in an enlarged fashion and a projection display device.
2. Description of the Related Art
Much attention is now focused on a projection projector system (projection display device) using a DMD display device as the light valve.
In the DMD, a fine rectangular mirror (mirror element) which has high reflectivity and which can be inclined in a range of 10 degree or more in accordance with a video signal, on a silicon memory chip by means of the CMOS semiconductor technology. The projection projector system using the DMD can control a reflection direction of a light emitted from a light source by varying an angle of the mirror element in such a way that only a desired reflected light can be converged on a screen to project a desired video thereon.
In this DMD, for example, a few or more million mirrors are aligned in a matrix manner on a substrate so that the huge number of mirrors can be digital-controlled independently. Therefore, each mirror corresponds to one pixel in a video.
Also, since there is no need to polarize an illumination light unlike a liquid crystal display device, the loss of light is small. Also, this DMD is excellent in the accuracy of a gradation representation.
Meanwhile, in the projection projector system using the DMD, the illumination system is configured without the prism, which is used for color synthesis and separation of illumination light/projection light, being disposed on a reduction side of the projection lens. That is, the so-called single board time-division system is often employed. In this case, a space in which the prism is arranged can be omitted and also there is no need to construct the reduction side of the projection lens as the telecentric system. Therefore, it is required that a further reduction in size of the lens should be achieved by setting eye on the reduction side in a position close to the panel. In addition, a high picture quality that conforms to a resolution of the device is demanded. In this case, a wider angel of view and a zooming with a high zoom ratio are needed from a viewpoint of the installation performance.
JP 2004-271668 A (corresponding to U.S. Pat. No. 6,888,682) discloses a zoom lens system, which has wider angle of view/high zoom ratio and can satisfy such requests to some extent.
However, in the technology disclosed in JP 2004-271668 A, the optical performances are satisfactorily maintained by using plural aspheric lenses, as apparent from the description of the respective embodiments. As a result, a too heavy burden is laid on the processing and assembling of lenses and increase in production cost is caused. In other words, it can be said that JP 2004-271668 A can maintain the optical performances satisfactorily only when using plural aspheric lenses.
Also, the demand for the lateral color becomes severe more and more to satisfy the higher definition of the imaging device. The zoom lens system disclosed in JP 2004-271668 A leaves room for improvement as to the lateral color.